1. Field of the Invention
The present invention relates to a billet for cold forging with high deformability and maintained hardenability, which can continuously be cold-forged without the need for process annealing, a technique for manufacturing such a billet for cold forging, a method of continuously cold-forging such a billet into a disk-shaped part with a shaft such as a connecting rod for an engine, and a cold-forging die apparatus.
2. Description of the Related Art
Heretofore, crankshafts and connecting rods for use in engines for motorcycles have mainly been manufactured by a hot forging process. It is the general practice to heat the material to a temperature equal to or higher than its recrystallization temperature and forge to shape.
Materials for use in the hot forging process include a thermally refined steel and a thermally unrefined steel. The thermally refined steel is a steel that has been heated to about 1200° C. and thereafter quenched and tempered for increased strength and toughness. A carbon steel that is used as the material of crankshafts is usually treated with heat.
The thermally unrefined steel is a steel with added vanadium that has been heated to about 1200° C. and thereafter air-cooled for increased strength and toughness.
Crankshafts have portions, i.e., worms and tapers, that are required to have a higher hardness than other portions thereof. The crankshafts need to contain carbon (C) in order to increase the hardness of these portions by subsequent induction hardening. Therefore, the material which is to be hot-forged into crankshafts is usually a carbon steel according to JIS S48C.
The carbon steel S48C is composed of 0.45–0.51 wt % of C, 0.15–0.35 wt % of Si, 0.6–0.9 wt % of Mn, 0.03 wt % or less of P, 0.035 wt % or less of S, 0.3 wt % or less of Cu, 0.2 wt % or less of Ni, and 0.2 wt % or less of Cr.
The carbon steel S48C has such cold forgeability that its upsetting ratio ranges from about 70 to 75%. If the carbon steel S48C were cold-forged at an upsetting ratio of 90% or more, then it would crack and fail to be deformed to desired shape.
The defonnability of the carbon steel S48C is affected by elements including Si, F, S, and Cu. Si is effective to increase the hardness and tensile strength of the steel and speed up the growth of crystal grain upon heat treatment, but tends to reduce stretchability and impact values for thereby impairing the forgeability of the steel. P in the form of a solid solution in ferrite also increases the hardness and tensile strength of the steel, but is liable to reduce impact values, making it easy for the steel to crack and cause cold brittleness. If the carbon steel S48C contains a large amount of S, then it precipitates manganese sulfide (MnS) that tends to start cracking when the steel is cold-forged, so that the steel is apt to crack when machined. If the carbon steel S48C contains a large amount of Cu, then the ferrite hardness increases to the extent that impairs the cold forgeability of the steel.
From the standpoint of keeping a desired level of hardenability, it is preferable to contain the same amount of C as with the above material for hot forging. Mn is also desirable to be contained in the same amount as with the above material for hot forging because Mn in the form of a solid solution in ferrite lowers the transformation temperature of the steel, allowing the steel to be easily quenched.
The hot-forging process is disadvantageous in that since die surfaces used tend to be easily worn, hot-forged products have poor dimensional accuracy and need to have a large finishing allowance to be removed when they are machined, and hence are machined with low efficiency. Furthermore, because hot-forged products have a large lathing allowance, the number of lathes required to machine the hot-forged products increases, resulting in a large amount of initial investments.
In addition, inasmuch as the material is hot-forged after it has been heated, scales are produced from the material during the hot-forging process. Furthermore, the hot-forging process makes it difficult to keep the working environment clean because the die surfaces need to be coated with a parting agent.
The cold-forging process is capable of solving the above problems with respect to the dimensional accuracy of forged products, the working environment, and the initial investments. However, the greatest problem of the cold-forging process is that the deformability of the material that is cold-forged is so small that the material tends to crack during the cold-forging process.
One conventional cold-forging process for manufacturing a crankshaft is shown in FIG. 25 of the accompanying drawings.
As shown in FIG. 25, the conventional cold-forging process is carried out by slowly cooling a rolled billet to soften the billet, deep-drawing and upsetting the billet while it is cold, thereafter softening the billet at an intermediate stage to remove strains introduced by the deep-drawing and upsetting steps, roughly shaping the billet into a crankshaft while it is cold, finally shaping the crankshaft while it is cold, removing an outer circumferential edge of the crankshaft while it is cold, forming a pin hole in the crankshaft while it is cold, and thereafter finishing the crankshaft by grinding the shank thereof and induction-hardening the crankshaft.
The conventional cold-forging process is liable to cause the billet to crack in the upsetting stage more frequently than the hot-forging process. To prevent the billet from cracking, the billet is softened in the intermediate stage to cancel strains developed so far in the cold-forging process. If the billet tends to be deformed greatly, then it is necessary to add more softening steps in the intermediate stage.
With softening steps introduced in the intermediate stage, however, the cold-forging process which would otherwise be continuous except for die changing is interrupted, and heat-treatment apparatus need to be installed to operate somewhere in the entire cold-forging process. Consequently, the cold-forging process is also problematic though the problems are not as serious as those of the hot-forging process.
It is an object of first and second inventions to provide a composition for a billet for cold forging which can continuously be cold-forged, i.e., does not need an intermediate softening step in the cold-forging process, and can well be hardened, and a method of manufacturing such a billet for cold forging.
An object of a third invention is to provide a method of manufacturing such a billet for cold forging with a simplified spheroidizing step required to manufacture a billet that can continuously be cold-forged.
In the manufacture of crankshafts for use in engines for motorcycles, there is a process of forming a split-type crankshaft in the form of a disk with shafts attached to both sides thereof, and connecting the split-type crankshaft with a pin. One conventional process of forming such a split-type crankshaft is disclosed in Japanese laid-open patent publication No. 58-215237, for example.
According to the disclosed process, a round bar is forged into a disk-shaped crank body with a shaft, and a counterweight is formed separately from the disk-shaped crank body. Then, the counterweight is integrally joined to the disk of the crank body.
However, the above process is disadvantageous in that the number of steps of the process is large because the crank body and the counterweight are separately formed and then joined to each other.
Furthermore, since the joining step is different from the respective steps of forming the crank body and the counterweight, the joining step needs a preparatory action for joining the crank body and the counterweight to each other.
If the crank body and the counterweight are produced by hot forging, then a grinding process for removing scales from the crank body and the counterweight and a machining process for achieving a desired level of dimensional accuracy are subsequently required. Therefore, the efficiency with which to produce the crank body and the counterweight is relatively low, resulting in a low yield of crankshafts.
It is an object of a fourth invention to provide a method of cold-forging a crankshaft without a machining step to remove scales for thereby increasing a yield and achieving a large cost reduction.
Objects of fifth, sixth, and seventh inventions are to provide a method of cold-forging a disk-shaped part with a shaft, a method of cold-forging a crankshaft, and a method of cold-forging a disk-shaped part with a shaft, respectively, to manufacture a disk shaped part with a shaft which has disk portions of different volumes, by eliminating different steps of forming the disk portions of different volumes separately and joining them to each other, for thereby doing away with the trouble of a preparatory action and increasing a yield.
Japanese laid-open patent publication No. 60-102245 discloses a known apparatus for forging crankshafts.
The disclosed forging apparatus forms splines on the shank of a split-type crankshaft by forging. Specifically, the shank of a split-type crankshaft whose overall shape has been formed by hot forging is set in a lower die, and the counterweight of the split-type crankshaft is pressed by a nib of an upper die to form splines on the shank with a tooth die in the lower die. The nib is divided into separate components, and a resilient member is held against rear surfaces of the nib components. Even when the nib presses a counterweight having a step, the resilient member applies uniform forces the counter-weight around the shank to prevent shank side end surfaces from being displaced.
Although the cold-forging process can solve the problems with respect to the dimensional accuracy of forged products, the working environment, and the initial investments, the cold-forging process is costly in that it places large burdens on dies and hence the dies have a relatively short service life. The nib that is divided into the components to accommodate the height of the counterweight is only effective to handle the existing counterweight, but not applicable to different counter weights that are to be produced.
It is an object of an eighth invention to provide a cold-forging die apparatus which will solve the above problems.